This invention relates to a radiation-sensitive record disc, such as an optical disc or video disc, in which information recorded on a recording layer can be read by directing a light beam at the recording layer.
One conventional radiation-sensitive record disc employing a "sandwich" construction is shown in FIGS. 1A and 1B, which illustrate the disclosure in U.S. Pat. No. 4,074,282 issued to Balas et al. This record disc consists of two discs 2A and 2B, each of which may be formed of a transparent plastic plate. Optical information recording layers 1A and 1B, which consist of an energy-absorbing layer or a light reflective layer, are formed directly on one surface of each disc 2A, 2B. Discs 2A, 2B face each other so that recording layers 1A and 1B are positioned therebetween. Each disc 2A, 2B is bonded with adhesive agent S to concentric inner spacer 3 and concentric outer spacer 4 disposed therebetween to provide air space 5 between each recording layer 1A, 1B.
The discs of the above conventional record disc must be formed with a relatively small thickness since the recording/reading beam must pass through the discs without distortion. That is, the use of a disc having small thickness prevents beam energy loss and refraction of light as the beam passes through the disc. However, due to the disc's thickness and transparency requirements, the material used lacks stiffness. Thus, when the air or gas within the space between the discs expands or contracts as the surrounding heat or atmospheric pressure changes, the radiation-sensitive record disc bends or undulates in accordance with this expansion or contraction. Any resulting bending or undulation then causes defocusing of the reading/writing beam which produces inaccuracies in processing the information stored on the recording layers. In extreme cases, the record disc and the optical head will eventually contact each other thereby preventing any processing of the stored information.
The above difficulties led to the development of a radiation-sensitive record disc such as shown in FIG. 2. Reinforcing disc 6 having a rigidity at least equal to that of discs 2A and 2B is joined to discs 2A and 2B to strengthen the record disc. Concentric inner spacers 3A, 3B and concentric outer spacers 4A, 4B separate the discs and provide air spaces 5A and 5B in which recording layers are formed.
However, in the latter case, other difficulties arise because the properties of discs 2A, 2B and reinforcing disc 6 are different and dimensional differences occur. In response to temperature and/or humidity changes, expansion or contraction of discs 2A, 2B and reinforcing disc 6 tends to result in serious warping problems.